Yes. Most phones reserve 2 bytes per letter if you use unicode charactes. So when you use them, SMS has 80 instead of 160 letters. This issue is not N900 specific.

BTW, as far as I have seen, N900 is the only phone that imidiately and correctly calculates nuber of remaining characters. Most phones will say you are still typing within 1 SMS, and send 2 of them.